1Z0-830 sample questions

Question 1

What does LocalDate.of(2024, 1, 31).plusMonths(1) return?

Accepted Answer

2024-02-29. plusMonths adds the months and then, if the day-of-month is invalid for the resulting month, adjusts it to the last valid day; 2024 is a leap year, so the result is 29 February. Rolling over into March is the tempting wrong answer, but LocalDate clamps to the end of the month instead of overflowing.

Question 2

String s = null;
switch (s) {
    case "a" -> System.out.print("a");
    default -> System.out.print("other");
}

What happens when this code runs with s set to null?

Accepted Answer

A NullPointerException is thrown. A switch without a case null label throws NullPointerException when the selector is null; the default label does not match null. To handle null, Java 21 lets you add case null or case null, default. The compile-error option is wrong because null checking is a run-time matter and the code is valid.

Question 3

Which member causes a compile-time error when added to the body of record Point(int x, int y)?

Accepted Answer

private int cachedHash;. A record's state is fixed by its components, so it cannot declare instance fields; static fields, instance methods and nested types are all allowed. The static field is the tempting wrong answer, but static members do not add per-instance state and are permitted.

Question 4

class Res implements AutoCloseable {
    String n;
    Res(String n) { this.n = n; System.out.print("open" + n + " "); }
    public void close() { System.out.print("close" + n + " "); }
}
try (Res a = new Res("A"); Res b = new Res("B")) {
    System.out.print("body ");
    throw new RuntimeException();
} catch (RuntimeException e) {
    System.out.print("catch ");
} finally {
    System.out.print("finally");
}

What is printed?

Accepted Answer

openA openB body closeB closeA catch finally. Resources are closed in the reverse order of their declaration, and they are closed before any catch or finally block of the try-with-resources statement runs. Placing catch before the close calls is the common mistake; the resources are already closed when the handler executes.

Question 5

What does Collections.sort produce for a mutable list containing banana, Apple, cherry and apple?

Accepted Answer

[Apple, apple, banana, cherry]. String's natural ordering compares UTF-16 code units, and uppercase letters (A is 65) sort before lowercase ones (a is 97), so Apple comes first and then the lowercase words alphabetically. [Apple, banana, cherry, apple] wrongly places apple last, but a-p-p sorts before b-a-n and c-h-e.

Question 6

What does collecting an empty Stream<String> with Collectors.partitioningBy(s -> s.length() > 1) return?

Accepted Answer

{false=[], true=[]}. partitioningBy always returns a map containing both the false and true keys, each mapped to the downstream result, even when no elements exist. {} is the answer for groupingBy, which only creates keys for groups that actually occur.

Question 7

Which command compiles every module under src, where each module has its own subdirectory named after the module, into separate module directories under out?

Accepted Answer

javac -d out --module-source-path src -m com.acme.api,com.acme.app. In multi-module mode, --module-source-path locates each module's sources in a subdirectory named after the module, -m selects the modules to compile, and -d is required to receive one output directory per module. Omitting -d is the tempting slip, but javac requires an output directory in multi-module mode.

Question 8

What does declaring a field volatile guarantee?

Accepted Answer

A write by one thread is visible to subsequent reads of that field by other threads. volatile establishes a happens-before relationship between a write and later reads of the same variable, preventing stale reads. It does not make count++ atomic, because that is still a separate read and write that another thread can interleave.

Question 9

What does Path.of("/a/b/../c/./d").normalize() return?

Accepted Answer

/a/c/d. normalize removes . elements and removes each .. together with the non-.. name element before it, so b/.. cancels out. /a/b/c/d is tempting if you think only . is removed; normalize is purely syntactic and does not check the file system.

Question 10

What does NumberFormat.getInstance(Locale.GERMANY).parse("1.234,5") return?

Accepted Answer

1234.5. German formatting uses a full stop for grouping and a comma as the decimal separator, so "1.234,5" parses to the number 1234.5. Parsing with Locale.US would stop at the comma and return 1.234.

1Z0-830 sample questions with answers

1Z0-830 sample questions

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